Synthesis, Crystal Structure of Tetra-Nuclear Macrocyclic Zn(II) Complex and Its Application as Catalyst for Oxidation of Benzyl Alcohol

A new six coordinated tetra-nuclear macrocyclic Zn(II) complex, ZnL4(Phen)2 (1) (HL= 3-bromo-2hydroxybenzaldehyde-pyridine-2-carbohydrazone, Phen = 1,10-phenanthroline) has been synthesized by the selfassembly of 3-bromo-2-hydroxybenzaldehyde-pyridine-2-carbohydrazone, Zn(CH3COO)2•2H2O, NaOH and 1,10phenanthroline in water/ethanol (v:v = 1:3) solution. Complex 1 was characterized by elemental analysis, infra red (IR), and single-crystal X-ray diffraction (XRD) analysis. The results show that Zn1 and Zn1b ions are sixcoordinated with a distorted octahedral geometric configuration by four O atoms of two different L ligands and two N atoms of two different L ligands, Zn1a and Zn1c ions are also six-coordinated with a distorted octahedral geometric configuration by two N atoms of two different L ligands, two N atoms of Phen ligands and two O atoms of two different L ligands. Complex (1) forms 3D network structure by the interaction. The selective oxidation reactions of benzyl alcohols catalyzed by complex (1) was investigated. The highest benzyl alcohol conversion and benzaldehyde selectivity were obtained at 100 °C for 4 h under 5 bar of O2. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).


Materials and Measurements
T h e l i g a n d o f 3 -b r o m o -2h y d r o x y b e n z a l d e h y d e -p y r i d i n e -2carbohydrazone was synthesized by our research group itself and confirmed by element analysis and infrared spectroscopy. Other reagents of NaOH (A. R.), 1,10-phenanthroline (A. R.), and Zn (CH3COO) 3-Bromo-2-hydroxybenzaldehyde-pyridine-2carbohydrazone (0.5 mmol, 0.11601 g) and NaOH (0.5 mmol, 0.020 g) were dissolved into 20 mL ethanol/H2O solution (v:v = 1:1) with stirring at room temperature. After 0.5 h, Zn(CH3COO)2•2H2O (0.5 mmol, 0.0865 g) was added to the above solution. After reaction at ca. 75 °C for 3 hours, 1,10-phenanthroline (0.5 mmol, 0.0901 g) was added to the above solution. The reaction mixture was stirred for 4 h at ca. 75 °C. The white precipitate formed and filtered. Slowing volatilization of filtrate at room temperature, the crystals of tetra-nuclear macrocyclic Zn(II) complex (1) were obtained in 30 days. Elemental analysis (%) calcd. for C19H12BrN4O2 Zn: C, 48.14; H, 2.53; N, 11.82. Found (%): C, 47.96; H, 2.76; N, 11.59.

Crystal Structure Determination
A suitable (0.12 mm × 0.11 mm × 0.09 mm) single crystal of tetra-nuclear macrocyclic Zn(II) complex (1) was selected to collect data on a Super Nova, Dual, Cu at zero, AtlasS2 diffractometer. The crystal was kept at 100.00(10) K during data collection. SHELXL program [29] was used to solve the structure by direct method, and refined by the OLEX2 program [30]. The crystallographic data of tetra-nuclear macrocyclic Zn(II) complex (1) are given in Table 1.

The Procedure for The Oxidation of Benzyl Alcohol
The selective oxidation reactions of benzyl alcohols catalyzed by tetra-nuclear macrocyclic Zn(II) complex (1) was carried out in a 20 mL stainless steel high pressure reactor. 0.050 g tetra-nuclear macrocyclic Zn(II) complex (1) catalyst, benzyl alcohol (1.0 mmol) and 1,4dioxane (7 mL) were added into the reactor, and then pure O2 was purged into the reactor. The suspension was stirred magnetically at 90 °C or 100 °C for 4 h under 3-5 bar O2 pressure. The conversion of benzyl alcohol and the selectivity of benzaldehyde were determined by gas chromatography spectrometer equipped with a SE-54 column. The products were identified by comparison with known authentic standards, and an external standard method was used for the qualitative analysis. (1) The structural analysis of tetra-nuclear macrocyclic Zn(II) complex (1) shows that it crystallizes in the orthorhombic system with the Fddd (no. 70) space group. The molecular structure of tetra-nuclear macrocyclic Zn(II) complex (1) is shown in Figure 1. The selected bond lengths (Å) and angles (°) for tetranuclear macrocyclic Zn(II) complex (1) are given in Table 2. As shown in Figure 1, the tetranuclear macrocyclic Zn(II) complex (1) is made up of four Zn(II) ions, four 3-bromo-2-   -N), respectively, which are comparable to other Zn(II) complexes [31][32][33]. The tetranuclear macrocyclic Zn(II) complex (1) assemble an extended 3D supramolecular network matic rings of ligands ( Figure 2

Activity of Benzyl Alcohol Oxidation
After preparation and characterization of tetra-nuclear macrocyclic Zn(II) complex (1), its catalytic activity was investigated in the selective oxidation of benzyl alcohol with molecular oxygen as the sole oxidant. O2 is inexpensive and only produces water as its byproduct. The reaction temperature and pressure were optimized in the selective oxidation of benzyl alcohol in the presence of tetra-nuclear macrocyclic Zn(II) complex (1). The results are summarized in Table 3. A blank experiment showed a low benzyl alcohol conversion (9.6%) at 100 °C under 5 bar of O2 with 1,4-dioxane as solvent for 4 h. By contrast, good benzyl alcohol conversions were observed tetra-nuclear macrocyclic Zn(II) complex (1), suggesting that complex (1) could catalyse the oxidation of benzyl alcohol. For tetra-nuclear macrocyclic Zn(II) complex (1), the benzyl alcohol conversion and benzaldehyde selectivity were 37.1% and 5.2% at 90 °C for 4 h under 5 bar O2. When the reaction temperature increase to 100 °C, the benzyl alcohol conversion and benzaldehyde selectivity greatly enhanced to 78.1% and 29.2%. The conversion of benzyl alcohol and selectivity of benzaldehyde were 49.0% and 10.8% at 100 °C for 4 h under 3 bar O2. The observed main by-product is benzoic acid, together with amounts of benzyl benzoate. The selectivities of benzoic acid are 55.3%, 45.4%, 47.3% for complex (1) on oxidation reactions under 90 °C 5 bar, 100 °C 5 bar, and 100 °C 3 bar, respectively. The highest benzyl alcohol conversion (78.1%) and benzaldehyde selectivity (29.2%) were obtained at 100 °C for 4 h under 5 bar of O2. Nabae et al. found that HBPI (hyperbranched polyimide) functionalized with TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) could works as a heterogenous catalyst for the benzyl alcohol oxidation in the presence of a catalytic amount of HNO3 [34].
The benzyl alcohol conversion and benzaldehyde selectivity are 11% and 100% on TEM-PO/HBPI, respectively [34]. Li et al [35]. reported that conjugated metalloporphyrin polymers (MnP-AMPs) with BET surface area of 345 m 2 /g had good catalytic performance for the oxidation of benzyl alcohol, achieving complete conversion within 2 h and benzaldehyde selectivity of 98%. Asgharnejad et al. [36] synthesized three-dimensional copper-based coor- ( 1 , 4 -B D C -B r ) , CH3COOH, and DMF. The copper-based coordination polymers exhibited good activity (conversion: 38%) and selectivity to benzaldehyde (78%) in the benzyl alcohol oxidation using tert-butyl hydroperoxide as an oxidant in DMF at 40 °C for 4 h [36]. Based on the above results, the catalytic activity and selectivity of complex (1) in the oxidation of benzyl alcohol was lower than MnP-AMPs catalyst. Although the selectivity of benzaldehyde is less than those of TEMPO/HBPI and [Cu(1,4-BDC-Br)(DABCO)0.5]·xDMF·yH2O, the complex (1) could oxidized benzyl alcohol with high activity using O2 as sole oxidant without adding any other substances.
To examine the scope of substrate of the oxidation reaction, we extended our studies to different combinations of alcohols.  Table 3. Oxidation of benzyl alcohol to benzaldehyde on tetra-nuclear macrocyclic Zn(II) complex (1) in 1,4-dioxane.

Conclusions
In summary, a new tetra-nuclear macrocyclic Zn(II) complex, ZnL4(Phen)2 (1) has been synthesized and structural characterized by elemental analysis, IR, and single-crystal X-ray diffraction analysis. The selective oxidation reactions of benzyl alcohols catalyzed by complex (1) have been investigated.